Molded sponge-like thermoplastic elastomer articles having thin, dense skins and a process for their production

ABSTRACT

A molded object having a sponge-like interior surrounded by a skin, molding composition from which it is produced, and the method for producing the article. Copolymers of vinyl-substituted aromatic compounds containing from 8-18 carbon atoms copolymerized with conjugated diene having from 4-12 carbon atoms and polymer chosen from among solid resinous polymers of vinyl-substituted aromatic compounds present in amounts ranging from about 10 to about 75 parts by weight per 100 parts by weight of the copolymer and solid resinous copolymers of ethylene present in an amount of about 5 to about 50 parts by weight per 100 parts by weight of the copolymer are blended with a sufficient amount of polarizing agent to achieve rapid heating through response to microwave treatment, blowing agent in an amount sufficient to provide a sponge-like, rubbery porosity to an article molded therefrom, and, optionally, cellular control agent. Composition is melted at a temperature that activates the blowing agent, molded, and removed from the mold as an article having a sponge-like interior surrounded by polymeric skin.

This application is a continuation-in-part of a copending applicationhaving Ser. No. 134,829, filed Mar. 28, 1980 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to molding expandable rubbery compositions andarticles molded therefrom. In one of its aspects, this invention relatesto a process for molding sponge like articles. In still another of itsaspects, this invention relates to the use of microwave energy inmolding expandable thermoplastic elastomer compositions.

In the molding of compositions based on the combination of athermoplastic block copolymer elastomer and a normally solid resinouspolymer combined with the polarizing agent in sufficient amount toachieve rapid heating through response to microwave treatment, it isknown that an article having a skin and a porous interior can beproduced by the addition of blowing agent to the molding compositionwith subsequent melting and molding at temperatures sufficient toactivate the blowing agent. It has now been discovered that bycontrolling the amount of blowing agent present in the moldingcompositions the porosity and skin thickness of the molded article canbe controlled so that articles can be produced having relatively thinskin surrounding a sponge-like interior which are suitable for use inservices where a soft feel and easy bending are desirable such asmetatarsal supports.

In these products, structural integrity of the polymeric wallssurrounding the interstitial voids is of great importance so that theshape of the molded article can be maintained. To maintain the loadbearing strength of the cell walls, it has been found necessary whenrelatively large pores are produced in the blown, molded objects tointroduce stabilizing compounds into the molding compositions tomaintain the structural stability of the molded objects.

It is therefore an object of this invention to provide a moldedpolymeric material in which a central portion of sponge-like appearanceis surrounded by a thin skin of highly dense material. It is anotherobject of this invention to provide a method for preparing a moldedarticle in which a thin skin of highly dense material surrounds asponge-like interior. It is still another object of this invention toprovide a molding composition and form from such composition a moldedobject having a thin skin surrounding a sponge-like interior.

Other aspects, objects and the various advantages of this invention willbecome apparent upon reading this specification and the appended claims.

STATEMENT OF THE INVENTION

According to this invention a molding composition is provided from whicha molded article comprising a thin, dense skin surrounding asponge-like, rubbery interior can be produced. The composition comprises(1) vinyl-substituted aromatic compounds containing from 8-18 carbonatoms copolymerized with conjugated diene having from 4-12 carbon atoms,(2) resinous, solid polymer of vinyl-substituted aromatic compound inthe range of about 10 to about 60 parts resinous polymer per 100 partscopolymer from (1), (3) polarizing agent, and (4) blowing agent in anamount sufficient to provide a sponge-like, rubbery porosity to anarticle molded therefrom.

In an embodiment of the invention a method is provided for producing amolded object having an interior of rubbery, sponge-like porositysurrounded by a thin dense skin. In this method a molding composition asdescribed above is introduced into a mold, melted with microwave energyat a temperature sufficient to activate the blowing agent, subjected tomolding conditions, and removed from mold as a molded article.

In another embodiment of the invention a method is provided for moldingarticles in which (1) vinyl-substituted aromatic compounds containingfrom 8-18 carbon atoms copolymerized with conjugated diene having 4-12carbon atoms, (2) resinous, solid polymer of vinyl-substituted aromaticcompound in the range of about 10 to about 60 parts resinous polymer per100 parts copolymer, (3) polarizing agent, and (4) blowing agent in anamount sufficient to provide a sponge-like porosity to the interior ofthe molded object are blended into a molding composition, (b) themolding composition is introduced into a mold, (c) melted with microwaveenergy at a temperature sufficient to activate the blowing agent, (d)subjected to molding conditions, and (e) removed from the mold as amolded article. The molded article produced by this method illustratesyet another embodiment of the invention.

Thermoplastic rubber sponge products are formed in the process of thisinvention by employing compositions containing as the rubber(s) anormally solid thermoplastic elastomer. The compositions are heatedsoftened for subsequent molding in a microwave energy field. Low moldingpressures, that is molding pressures below about 200 psi (1.4 MPa) arecharacteristically utilized in forming the products that are made in theprocess of the instant invention.

For the purpose of this invention the microwave frequencies which can beemployed range from about 10 MH_(z) to about 100 MH_(z). Of particularinterest are the frequencies of 12 MH_(z) and 40 MH_(z) since largepower tubes, e.g., klystrons, magnetrons, are available for convertingdirect current into microwave energy.

Suitable thermoplastic elastomers in this invention are normally solid,block copolymers which exhibit high tensile strength and elongation intheir unvulcanized (green) state. The block copolymers can be formed bycopolymerizing a vinyl-substituted aromatic compound containing from 8to 18 carbon atoms with a conjugated diene having from 4 to 12 carbonatoms as described in U.S. Pat. No. 3,281,383 which issued to R. P.Zelinski and H. L. Hsieh.

Paritcularly suitable block copolymer usefully employed in thisinvention are the linear block or radial teleblock copolymers ofbutadiene and styrene. Such copolymers are more completely described inU.S. Pat. Nos. 3,823,109; 3,826,776; and 3,959,545, incorporated here byreference.

These butadiene/styrene block copolymers generally contain from about 10to about 50 weight percent polymerized styrene and about 90 to about 50weight percent polymerized butadiene, preferably from about 20 to about50 weight percent polymerized styrene and from about 80 to about 50weight percent polymerized butadiene. The block copolymers generallypossess a weight average molecular weight in the range from about 75,000to about 500,000 with a range from about 100,000 to about 350,000 beingmore preferable. A mixture of such polymers can be employed to modifymelt flow, stiffness, and like characteristics, if desired.

Other components of the sponge-forming compositions of this inventioninclude normally solid resinous polymers of vinyl-substituted aromaticcompounds, e.g., styrene, alpha-methyl styrene, alone or copolymerizedwith a monomer such as acrylonitrile or with minor amounts of aconjugated diene such as 1,3-butadiene. These polymers, which can beselected to increase the melt flow of the compositions, generally have adensity ranging from about 1.04 to about 1.10 g/cc and a tensilestrength ranging from about 5,000 to about 12,000 psi (34.5-82.7 MPa),ASTM D 638, and a Shore A hardness ranging from about 35 to about 95(ASTM D 2240) at 23° C. Such polymers are generally used in amountsranging from about 10 to about 75 parts by weight per 100 parts byweight block copolymer. Other normally solid resinous polymers which canbe employed also include copolymers of ethylene with monomers such asethyl acrylate, vinyl acetate, and the like. Polymers of this type canimprove the response of the final composition to the effects ofmicrowave energy as well as improving the melt flow of the compositions.Suitable copolymers of this class are selected so that their meltingpoints range from about 200° to about 250° F. (93°-121° C.). Thesepolymers are generally employed in amounts ranging from about 5 to about50 parts by weight per 100 parts by weight block copolymer.

Polarizing agents are included in the compositions employed in theprocess of this invention to insure that they will melt in a microwavefield. The agents are normally liquid or normally solid polar compoundsand are generally selected from among simple and polymeric alkyleneglycols and their mono and dialkyl ethers, ethanolamines andisopropanolamines and their hydrocarbyl-substituted derivatives andmixtures thereof. Specific examples of these compounds include ethyleneglycol, 1,2-propylene glycol, 1,3-butylene glycol, 1,4-butylene glycol,1,6-hexylene glycol, diethylene glycol, triethylene glycol, dipropyleneglycol, thiodiethylene glycol, etc., polyethylene glycols having averagemolecular weights ranging from about 200 to about 6,000; polypropyleneglyols having average molecular weights averaging from about 400 toabout 2,000; mixed poly(ethylene)-poly(propylene) glycols having averagemolecular weights ranging up to about 6,000 and containing from about 30to about 90 weight percent ethylene oxide; the monomethyl, monoethyl andnonobutyl ethers of ethylene glycol, propylene glycol and diethyleneglycol; the monomethyl and monoethyl ethers of triethylene glycol; thedimethyl and diethyl ethers of diethylene glycol, dipropylene glycol andtrimethylene glycol; the alkanolamines based on ethanol and isopropanolsuch as mono-, di- and triethanolamine, mono-, di- andtriisopropanolamine, methylethanolamine, dibutylethanolamine,phenyldiethanolamine, di(2-ethylhexyl)ethanolamine,dimethylisopropanolamine, dibutylisopropanolamine, and the like; andmixtures thereof. Other polar compounds such as liquid acrylonitrilebutadiene polymers, acrylonitrilebutadiene blends with homopolymers ofpolyvinylchoride and styrene acrylonitrile copolymers are effective.Other agents include glyceryl diacetate anddi(2-hydroxyethyl)-5,5-dimethylhydantoin. Particularly suitablepolarizing agents include triethanolamine and polyethylene glycol, e.g.,Carbowax® 540, totaling from about 3-8 parts by weight per 100 parts byweight thermoplastic elastomer.

Blowing agents, preferably chemical blowing agents are utilized in thecompositions at a level sufficient to produce an article which can beirregular in shape, which can exhibit a dense skin and a microporous tosponge-like interior and which can taper off to zero thickness as in thecase of metatarsal supports. The level of blowing agent or mixture ofagents to employ can range from about 3 to 10 parts by weight per 100parts by weight thermoplastic elastomer depending upon the desiredspecific gravity of the finished articles with the spongeness of thearticle increasing with the amount of blowing agent incorporated (otherconditions being the same). Preferably the amount of blowing agentranges from about 6 to about 10 parts by weight per 100 parts by weightthermoplastic elastomer to produce a sponge-like interior. Particularlysuitable compounds are azobisformamide (ABFA) and 4,4'-oxybis(benzenesulfonyl hydrazide) (OBSH) and it is presently preferred to usea mixture of these compounds wherein the weight ratio of ABFA/OBSH canrange from about 1:2 to about 4:1.

Other components used in preparing the compositions used in the processof this invention include odorants, colorants, fillers, plasticizingagents, processing aids, antioxidants and UV stabilizers. Suitablefillers include metal silicates, metal oxides, metal carbonates, and thelike such as calcium metasilicate, silica, alumina, titania, calciumcarbonate, clays, etc. Examples of plasticizing agents includenaphthenic petroleum oils, e.g., ASTM type 104A, esters of adipic acid,phthalic acid, etc. Processing aids include the metal stearates, e.g.,calcium stearate, zinc stearate, silicones, natural and synthetic waxes,and the like. Antioxidants and UV stabilizers can be added as desiredfrom suitable commercially available materials. Exemplary of theseinclude thiodipropionic esters, e.g., dilaurylthiodipropionate, hinderedphenolic antioxidants, e.g., 2,6-di-t-butyl-4-methyl-phenol,octadecyl[3-(3,5-di-t-butyl-4-hydroxyphenyl)]propionate, thiodiethylenebis(3,5-di-t-butyl-4-hydroxy) hydrocinnamate, etc., and UV stabilizerssuch as 2(2'-hydroxy-5'-methylphenyl) benzotriazole,2-hydroxy-4-n-octoxybenzophenone,[2,2'-thiobis(4-t-octyl-phenolato)]-n-butylamine-nickel(II), etc.Generally, the amounts of the various components in parts by weight per100 parts by weight thermoplastic elastomer (php) will be as follows:filler, 10 to 150 php; antioxidant, 0.1 to 1 php, and UV stabilizer, 0.1to 3 php.

The quantity of plasticizing agents useful in the instant inventionranges from about 20 to about 150 parts by weight per 100 parts byweight of thermoplastic elastomer. A portion or all of said agent can beincluded in those thermoplastic elastomers which are producedcommercially in the form of oil-extended polymers e.g., can contain 50parts by weight naphthenic oil (plasticizing agent) per 100 parts byweight thermoplastic elatomer.

In addition to the other components described before it is alsoadvantageous to include cellular control agents which supply sufficientstrength to the cell walls of highly porous molded articles to make thearticles structurally sufficient for practical use. These includenonionic surfactants comprising silicone compounds. For example, asilicone-glycol copolymer can be prepared from a long chain alkylsilicic ester such as the triethyl ester of octadecylsiliconic acid (1mole) by reaction with about 3 moles of polyethylene glycol containingfrom about 2 to about 12 repeating units of the --OCH₂ CH₂ -- group. Thecompounds are described in U.S. Pat. Nos. 2,465,307 and 2,476,308.Another suitable class of compounds comprise copolymers ofdimethylpolysiloxanes and polyoxyalkylene ethers as for example thecompound C₂ H₅ Si[O (Si(CH₃)₂ O)_(a) (CH₂ CH₂ O)_(b) (C₃ H₆ O)_(c) C₄ H₉]₃ where the organic portion is a mixed copolymer containing equalweights of ethylene and propylene oxides and terminates with a butoxygroup, the silicone base and each of the three organic branch chains arerepeated sufficiently to obtain molecular weights of about 1500.Compounds of this type are described in the 2nd Edition of theEncyclopedia of Chemical Technology 19, 554 (1969).

The amount of cellular control agent if employed, generally ranges fromabout 0.5 to 2 parts by weight per 100 parts by weight thermoplasticelastomer.

The molded articles are relatively soft materials which have arelatively dense skin and a microporous interior in the thickerportions. Thin portions, e.g., those about 10 mils or less in thicknessare generally dense. The average Shore A hardness of the molded articlesranges from about 20 to about 35 (ASTM D 2240-75).

The compositions are normally mixed together by employing compoundingmeans such as roll mills, Banbury mixers, extruders, high intensitymixers, e.g., Henschel mixer, and the like. All of the components can bemixed together at once or a concentrate blend of the sensitizingcomponents alone or with compatible ingredients can first be blended tomake a homogeneous mixture and subsequently incorporated into thethermoplastic elastomer that makes up the majority of the moldingcomposition. Maximum temperatures employed must be below thedecomposition temperature of any blowing agent present. Usually thecompounded formulation is converted into a slab of appropriatedimensions for the mold to be used or the formulation can be convertedinto a free flowing particulate form by using conventional grinding orpelletizing processes. Desirably, just enough of the particles of asuitably dimensioned preformed slab is used in the molding process toavoid excess flash. The flash and reject articles can be granulated andused as a portion of a mix also comprising virgin material. The weightof reground material in such a mix is preferably no more than about 20percent of the total when colored materials is used, in general, likecolors are blended together.

The molds employed in shaping the invention compositions are generallyconstructed from low cost, relatively low strength materials includingsilicone rubber, polysulfide rubber, (Ryton®) polyphenylene sulfide,polyurethane, plaster, cast aluminum, and the like. The nature of themold is dependent upon the molding process used. If the inventioncomposition is placed within the mold and the entire assembly ispreheated by microwave energy, it is preferred that the mold used bemade of a material such as silicone rubber that has a lower dielectricloss factor than the invention composition. It is within the scope ofthis invention to preheat the composition in a low dielectric losscontainer such as glass or ceramic and transfer it to a mold constructedfrom metals and the like for the actual shaping step. Generally, thecomposition is placed within a silicone rubber mold, the mold top iscovered with a silicone rubber sheet or a silicone release paper, e.g.,paper or the like covered with a release agent such as a silicone, andassembly is placed between the plates of a high frequency electricalfield which form a part of commercially available molding machines. Thetop plate is lowered to contact the release paper covering the mold andthe composition is preheated by application of microwave energy for adesired length of time. After preheating to softening temperature,sufficient pressure is employed to compression mold the composition,e.g., about 10 to about 200 psig (68.9-1380 kPa), for a period of timegenerally ranging from about 0.1 to about 10 times the preheating time.The pressure is released, the assembly is preferably placed in aseparate zone for cooling the mold and contents, after which the moldedarticle is removed. A rotary table or the like containing a plurality ofmolds can be employed to provide molded parts at commercially attractiverates. Separation of the heating and cooling zones speeds production andreduces consumption of power and water.

A suitable microwave unit for molding is a KN unit, Model J,manufactured by Compo-Industries, Inc., Waltham, Mass. This unit whichhas a 10 kilowatt output, is the one employed in the specific example,unless otherwise indicated.

EXAMPLE

A series of compositions was prepared by mixing the components in aBanbury mixer for 6 minutes at a temperature of about 138° C. Eachcomposition was dumped from the mixer at about 120° C. and granulated.The portion of the granulated material which passed through a screenhaving 1/4 inch (0.6 cm) openings was retained for molding purposes. Theformulations employed are shown in Table I and are given in parts byweight.

                  TABLE I                                                         ______________________________________                                         Thermoplastic Rubber Sponge Compositions                                     ______________________________________                                                         Composition                                                  Component          A       B         C                                        ______________________________________                                        Thermoplastic elastomer 1.sup.(1)                                                                150     0         0                                        Thermoplastic elastomer 2.sup.(2)                                                                0       150       150                                      Poly (alpha-methylstyrene).sup.(3)                                                               40      40        60                                       Crystal polystyrene.sup.(4)                                                                      25      25        0                                        Naphthenic extender oil.sup.(5)                                                                  50      50        50                                       Silica.sup.(6)     25      25        25                                       Stabilizer 1.sup.(7)                                                                             0.70    0.70      0.70                                     Stabilizer 2.sup.(8)                                                                             0.50    0         0                                        Zinc stearate      0       0         0.30                                     Blowing agent 1.sup.(9)                                                                          3       3         6                                        Blowing agent 2.sup.(10)                                                                         3       3         2                                        Triethanolamine.sup.(11)                                                                         3       4         4                                        Polyethylene glycol.sup.(12)                                                                     3       3         4                                        TiO.sub.2 (pigment)                                                                              0       1.4       0                                        Yellow pigment.sup.(13)                                                                          0.2     0         0                                        Blue pigment.sup.(14)                                                                            0       0.34      0                                        Total parts        303.40  305.44    302.00                                   Calculated specific gravity                                                                      1.06    1.06      1.00                                     ______________________________________                                        Samples Molded                                                                            A          B1       B2      C                                     ______________________________________                                        Machine Used                                                                             Commercial Model J  Model E                                                                              Model J                                 Sample made                                                                              mat with pro-                                                                            smooth   metatar-                                                                             metatar-                                           tuberances surface  sal    sal                                                           mat      support                                                                              support                                 Heating time,                                                                            50 est.    50       50 est.                                                                              30                                      seconds                                                                       Sample specific                                                                          0.65       0.60     0.55   0.50                                    gravity                                                                       ______________________________________                                         Notes:-                                                                       .sup.(1) Phillips Petroleum Co., Bartlesville, OK; radial teleblock 40/60     styrene/butadiene copolymer, weight average molecular weight of 240,000,      extended with 50 parts by weight naphthenic oil per 100 parts by weight       copolymer.                                                                    .sup.(2) Phillips Petroleum Co., radial teleblock 30/70 styrene/butadiene     copolymer, weight average molecular weight of 300,000, extended with 50       parts by weight naphthenic oil per 100 parts by weight copolymer.             .sup.(3) Amoco Chemical Corp., St. Paul, MN; linear homopolymer with          softening point of 240° F. (116° C.), ASTM D 36.                .sup.(4) Cosden Oil & Chemical Co., Big Spring, TX; Cosden® 500           Special.                                                                      .sup.(5) ASTM type 104A, Flexon® 766, Exxon Co., Houston, TX.             .sup.(6) PPG Industries Inc., Pittsburgh, PA; Hi Sil® 233,                Precipitated, hydrated, hydrated silica, 2.0 specific gravity, ultimate       particle size of 0.22 micron (2.2 × 10.sup.-5 mm).                      .sup.(7) Zinc dibutyldithiocarbamate.                                         .sup.(8) CibaGeigy Industrial Chemicals, Ardsley, NY; Tinuvin® P,         2(2'-hydroxy5'-methylphenyl) benzoatirazole.                                  .sup.(9) Uniroyal, Inc. Chemicals Div., Naugatuck, CT; Celogen® AZ,       azobisformamide.                                                              .sup.(10) Uniroyal, Inc. Celogen® OT, 4,4'-oxybis(benzenesulfonyl         hydrazide).                                                                   .sup.(11) Polarizing agent.                                                   .sup.(12) Polarizing agent, Union Carbide Corp., New York, NY;                Carbowax®  540, average molecular weight of about 540.                    .sup.(13) CibaGeigy Industrial Chemicals; Cd Lith Primrose 206.               .sup.(14) Custom Color, Inc. Lodi, OH; Microclean® 42,715 UC,             masterbatch containing 80 parts by weight blue pigment per 100 parts by       weight normally solid radial teleblock styrene/butadiene copolymer.      

Molded articles were made from the formulations given in the Table usingboth lab size and commercial size equipment. Formulation C represents anespecially suitable one. A metatarsal support was molded from it after a30 second heating period having a maximum thickness of about 1/4 inch(0.6 cm) tapering off to zero at the periphery, which exhibited goodinterconnecting cellular structure with good skin formation and anaverage specific gravity of 0.5.

We claim:
 1. A method for producing molded sponge like articlescomprising:(a) introducing into a mold an expandable thermoplasticelastomer composition comprising:(1) vinyl-substituted aromaticcompounds containing from 8-18 carbon atoms copolymerized withconjugated diene having from 4-12 carbon atoms, (2) resinous solidpolymer of vinyl-substituted aromatic compound in the range of about 10to about 60 parts resinous polymer per 100 parts said copolymer, (3)polarizing agent, and (4) blowing agent in an amount sufficient toproduce upon activation and molding an article of rubbery, sponge-likeporosity surrounded by a thin, dense skin, (b) melting said compositionwith microwave energy at temperatures sufficient to activate saidblowing agent and cause expansion of said melted composition, (c)shaping said expanding melted composition at low pressures to form saidsponge like molded article having a thin, dense skin; and (d) removingsaid molded article from the mold.
 2. A method of claim 1 wherein thetotal amount of blowing agent is in the range of about 6 to about 10parts by weight per 100 parts by weight thermoplastic polymer.
 3. Amethod of claim 3 wherein the blowing agent is a mixture ofazobisformamide (ABFA) and 4,4'-oxabis(benzenesulfonylhydrazide) (OBSH)in a weight ratio range of about 1:2 to about 4:1 of ABFA/OBSH.
 4. Amethod of claim 1 wherein the composition further comprises at least onecellular control agent.
 5. A method of claim 4 wherein the compositionfurther comprises a silicone-glycol copolymer as cellular control agent.6. A method for producing molded sponge like articles comprising(a)blending the followiang components to form an expandable thermoplasticelastomer molding composition: (1) vinyl-substituted aromatic compoundscontaining from 8-18 carbon atoms copolymerized with conjugated dienehaving from 4-12 carbon atoms, (2) resinous, solid polymer ofvinyl-substituted aromatic compound in the range of about 10 to about 60parts resinous polymer per 100 parts said copolymer, (3) polarizingagent, and (4) blowing agent in an amount sufficient to produce uponactivation and molding an article of rubbery sponge-like porositysurrounded by a thin, dense skin; (b) introducing said moldingcomposition into a mold; (c) melting said composition with microwaveenergy at temperatures sufficient to activate said blowing agent andcause expansion of said melted composition. (d) shaping said expandingmelted composition at low pressures to form said sponge like moldedarticle having a thin, dense skin; and (e) removing said molded articlefrom the mold.
 7. An article molded by the method of claim
 1. 8. Anarticle molded by the method of claim 2, 3, 4, or 5.